Dielectric and Wavefunction Engineering of Electron Spin Lifetime in Colloidal Nanoplatelet Heterostructures

Abstract Colloidal semiconductor nanoplatelets (NPLs) have emerged as low‐cost and free‐standing alternates of traditional quantum wells. The giant heavy‐ and light‐hole splitting in NPLs allows for efficient optical spin injection. However, the electron spin lifetimes for prototypical CdSe NPLs are within a few picoseconds, likely limited by strong electron‐hole exchange in these quantum‐ and dielectric‐confined materials. Here how this hurdle can be overcome with engineered NPL‐heterostructures is demonstrated. By constructing type‐I CdSe/ZnS core/shell NPLs, dielectric screening inside the core is strongly enhanced, prolonging the electron spin polarization time (τesp) to over 30 ps (or 60 ps electron spin‐flip time). Alternatively, by growing type‐II CdSe/CdTe core/crown NPLs to spatially separate electron and hole wavefunctions, the electron‐hole exchange is strongly suppressed, resulting in τesp as long as 300 ps at room temperature. This study not only exemplifies how the well‐established synthetic chemistry of colloidal heterostructures can aid in spin dynamics control but also establishes the feasibility of room‐temperature coherent spin manipulation in colloidal NPLs.

Synthesis of four monolayer (4 ML) CdSe NPLs. 4 ML CdSe NPLs were synthesized according to the literature, with slight modifications. 1,2 or a typical synthesis, 170 mg of cadmium myristate, 12 mg of Se, and 30 mL of octadecene (ODE) were loaded into a 50 mL three-neck flask.The solution was degassed and stirred at 95 o C under vacuum for an hour to evaporate volatile solvents and dissolve S2 the cadmium myristate completely.The heater was then set to 240 o C. When the temperature reached 100 o C, the flask was switched from vacuum to nitrogen gas.As the temperature reached 200 o C, the color of the solution became orange, and at this stage 40 mg of cadmium acetate dihydrate (Cd(Ac)2•2H2O) was introduced swiftly into the reaction.After the growth of CdSe NPLs at 240 o C for around 10 minutes, 1 mL of oleic acid (OA) was injected when the temperature of the solution was decreased to 160 o C using air gun.15 mL of Hexane was injected when the temperature is decreased to room temperature.The solution was centrifuged for 10 min at 3000 rpm, and the supernatant was transferred another centrifuge tube.This solution was centrifuged at 7800 rpm for 10 min, and the precipitates were dissolved and stored in toluene for further use.
Synthesis of CdSe/ZnS core/shell NPLs.CdSe/ZnS core/shell NPLs were synthesized following reported procedures with slight modifications. 3In a typical reaction, 0.2 mmol zinc acetate, 7 mL 4 ML CdSe NPLs (with optical density of 1.6 at 512 nm in 1 mm cuvette), 0.5 mL OA, and 5 mL ODE were added to a 50 mL three-neck flask.The solution was stirred under vacuum at room temperature for an hour to evaporate hexane.Subsequently, the mixture was heated up to 90 o C and kept for 30 min to completely remove water and/or other remaining volatile solvents.After the degassing step, 0.5 mL oleylamine (OLAm) was added into the solution under nitrogen flow.The reaction was then kept for 10 min and the temperature was set to 300 o C.An octanethiol-ODE solution (87.5 μL octanethiol in 5 mL ODE) was prepared in glove box and injected into the reaction at 170 o C using a syringe pump with a speed of 8.85 mL/h.When the temperature reached 250 o C, the injection speed of the octanethiol-ODE solution was changed to 3.54 mL/h.The reaction was kept at 300 o C for 60 min, after which it was quenched by injection of 5 mL ODE and an ice-water bath.The product was washed with 20 mL ethanol and centrifuged at 5500 rpm for 3 min.The precipitated final product was dispersed in hexane for optical characterization.CdSe/ZnS core/shell NPLs with different shell thicknesses were S3 obtained by changing the amount of OA and OLAm.By varying the amount of ligand from 0.02 to 1 mL, the shell thickness can be increased from 0.4 nm to 1.5 nm, as shown in Figure S1.The tellurium (Te) precursor was made by string a mixture of 1.7 mg Te powder, 0.4 mL Trioctylphosphine (TOP), and 5 mL ODE overnight in a glovebox.The Te precursor was then slowly injected using a syringe pump at a rate of 1 mL/hour.The reaction was quenched after ~1 hour and the product was washed by precipitation with acetone followed by centrifugation, and finally dispersed in hexane for optical characterizations.

Transient absorption
The femtosecond pump-probe TA measurements were based on a Pharos laser (1030 nm,100 kHz, 230 fs pulse-duration; Light conversion).One part of the Pharos output was used to pump an optical parametric amplifier (OPA; TOPAS) to generate the wavelength-tunable excitation pulses, while the other was sent through a delay stage and attenuated with a neutral density filter and focused into a 1 cm thick sapphire window to generate a white light continuum (WLC) used as the probe beam.The pump and probe beams were focused and overlapped onto the sample.Circularly polarized pump and probe pulse were generated separately by two sets of broadband polarizing beam splitter cubes (400-700 nm, Thorlabs) and quarter-wave plates S4 (350-850 nm, Thorlabs).Transverse magnetic fields were provided by an electromagnet (EM3; Beijing Jinzhengmao Technology Co.).

Estimation of exciton binding energy by fitting absorption spectra
We fit the absorption spectrum of CdSe/ZnS core/shell NPLs of varying shell thicknesses using a quantum well (QW) absorption model. 4,5 he excitonic absorption of a QW exciton is: where E0 is the absolute energy of the exciton, γX is the exciton line width and η is an asymmetric broadening factor.The corresponding absorption of the continuous band is: where HC is the step height of the continuum edge, γC is its width and Eb is the exciton binding energy.The absorption of the NPLs is the sum of excitonic and continuous absorptions of HH, LH and SO bands:

S9
As explained in the main text, the the ratio between the Eb of LH and HH excitons is fixed at 1.51 in all the fits.The fitting parameters are listed in Table S1.
CdSe/CdTe core/crown NPLs.The CdSe/CdTe core/crown NPLs were synthesized following procedures reported in literature with slight modifications.2 3 mL CdSe 4 ML NPLs (with optical density of 1 at 512 nm in 1 mm cuvette), 80 mg of Cd(Ac)2•2H2O, and 0.05 mL of OA was dissolved in 10 mL of ODE and degassed under vacuum for 30 min at 100 o C, and then heated to 190 o C under nitrogen flow.

Figure S8 .
Figure S8.TA kinetics of CdSe/CdTe NPLs measured with co-and counter-polarized pump/probe configurations probed near the CdSe HH bleach at (a) 514 nm and (b) 519 nm, as well as (c) near the CT bleach center at 630 nm.

Table S1 .
Fitting parameters for absorption spectra of CdSe/ZnS core/shell NPLs with varying shell thicknesses.

Table S2 .
Fitting parameters for the electron spin polarization decay of CdSe/ZnS core/shell NPLs with varying shell thicknesses.